1. Field of the Invention
The present invention relates to a roll stamper used when a substrate sheet for high-density information recording mediums such as optical disks and optical cards is produced by continuously transferring preformat patterns to the surface of a resin sheet. It also relates to an apparatus and a process for manufacturing a substrate sheet for information recording mediums by using the stamper.
2. Related Background Art
Processes for manufacturing substrates for high-density information recording mediums such as optical disks and optical cards are known to include, for example, processes such as injection molding, compression molding and extrusion. In the injection molding or compression molding, severe conditioning is required for the temperature of molten resin, pressure applied thereto, precision of a molds, etc. in order to assure flatness and smoothness of substrates and to prevent occurrence of warpage and inclusion of bubbles in substrates. In addition, an apparatus with a large scale is required, resulting in a high production cost. In the injection molding, it is also difficult to produce substrates as thin as 0.4 mm or less and having a low double refraction as in substrates for optical cards. Moreover, in the compression molding and injection molding, substrates are formed sheet-by-sheet and hence their productivity is poor.
As for the extrusion, substrates for information recording mediums can be manufactured by bringing a roll stamper provided with a stamper member, into pressure contact with the surface of a molten-resin sheet extruded from an extruder and continuously transferring patterns corresponding to preformats such as tracking grooves or prepits to the surface of the resin sheet. Hence, the extrusion is advantageous for the mass-productivity of substrates, and in recent years researches thereon have been put forward as a process for manufacturing substrates for information recording mediums.
Incidentally, roll stampars used in the extrusion, for molding substrates for information recording mediums are conventionally manufactured by the following methods.
(1) A method in which preformat patterns are directly formed in the periphery of a roll substrate whose periphery has been mirror-finished (hereinafter "specular roll substrate").
(2) A method in which a flexible stamper member with preformat patterns previously formed on its surface is uniformly stuck to the periphery of a specular roll substrate by means of an adhesive.
(3) A method in which a flexible stamper member with preformat patterns previously formed on its surface is screwed to the periphery of a specular roll substrate.
Of the above conventional techniques, the method (1) makes it necessary to change the whole roll stamper for new one even when only one fault is produced in preformat patterns, resulting in a poor molding efficiency. The roll stamper produced using the method (2) tends to undergo changes with time on the adhesive because of the heat of the resin sheet formed by extrusion or the heat of the roll stamper itself, and there is the problem that its adhesive force decreases to cause separation of the stamper from the specular roll, resulting in a very short lifetime of the roll stamper. There is another problem that bubbles tend to be formed in the adhesive and any irregularities caused by such bubbles are unwantedly transferred to the substrate, making it difficult to achieve scrupulous transfer of patterns to cause faulty transfer.
The roll stamper produced using the technique described in the method (3) may cause a strain in the flexible stamper member at the portions fastened with screws, or cause an uneveness at the screwed portions, i.e., the fastened portions, bringing about the problems that this uneveness or strain causes faulty transfer of preformat patterns to the molten-resin sheet and also this uneveness gives a non-uniform pressure applied to the molten-resin sheet to cause double refraction when, for example, a polycarbonate is used as the resin.
As a means for solving such problems, the present inventors have disclosed in Japanese Patent Application Laid-Open No. 3-97909 a roll stamper comprising a flexible stamper member provided with fixing members at both ends of its back surface, and a pair of fit grooves capable of fittingly receiving said fixing members, formed in the periphery of a roll substrate; said fixing members being so fitted to said fit grooves that said flexible stamper member is fixed along the periphery of the roll substrate between said fit grooves. This roll stamper has a constitution advantageous in that the fixing members have no influence on the state of the surface of the flexible stamper member. It, however, can not well stand against the flexible stamper member undergoing thermal expansion due to the heating of the roll stamper. In particular, a gap may be made between the flexible stamper member and the roll substrate when the roll stamper is so constituted as to have a large difference in elongation due to thermal expansion, between the flexible stamper member and roll substrate. This may give no smooth conduction of the heat having transferred from the molten-resin sheet to the flexible stamper member, to bring about the problem that a locally hightemperature region is produced on the surface of the roll stamper and besides the resin sheet may stick around the periphery of the roll stamper.